Yes / Seabed instability around a pipeline is one of the primary concerns in offshore pipeline projects. To date, most studies focus on investigating the wave/current-induced response within a porous seabed around either a fully buried pipeline or a thoroughly exposed one. In this study, unlike previous investigations, a series of comprehensive laboratory experiments are carried out in a wave flume to investigate the wave-induced pore pressures around a partially embedded pipeline in a trench layer. Measurements show that the presence of the partially buried pipeline can significantly affect the excess pore pressure in a partially backfilled trench layer, which deviates considerably from that predicted by the theoretical approach. The morphology of the trench layer accompanied with the backfill sediments, especially the deeper trench and thicker backfill (i.e.,b≥1D,e≥0.5D), provides a certain degree of resistance to seabed instability. The amplitude of excess pore pressure around the trench layer roughly exhibits a left-right asymmetric distribution along the periphery of the pipeline, and decays sharply from the upper layer of the trench to the lower region. Deeper trench depth and thicker buried layer significantly weaken the pore-water pressures in the whole trench area, thus sheltering and protecting the submarine pipeline against the transient seabed liquefaction. / The National Key research and development program of China (2017YFC1404200), the research grants of Jiangsu (BK20150804), the marine renewable energy research project of State Oceanic Administration (GHME2015GC01), Open Foundation of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (Project No: 2016491011), the Royal Academy of Engineering the Distinguished Visiting Fellowship (DVF1718-8-7)

Yes / Most previous studies on seabed liquefaction around offshore pipelines focused on investigating the wave-induced pore pressure variation in sandy seabed, while limited studies have been conducted for silty seabed. In this study, laboratory experiments are conducted to investigate wave-induced pore pressure within silty bed around the buried or partially/fully backfilled pipeline. Results show that residual pore pressure is the dominant factor that causes the liquefaction in silty soil. For buried pipeline, liquefaction first occurs at the pipeline bottom, then propagates upwards and downwards vertically. Comparing with the buried pipeline, the liquefaction potential is reduced when the pipeline is placed in a trench. To protect pipeline from liquefaction, backfill is recommended. Experiments show that the residual pore pressure significantly decreases as backfill depth increases. Fully backfilled pipeline is the best choice for silty seabed. Furthermore, backfill material with coarser particle size than native soil provides better protection for pipeline. In this study, there is no residual pore pressure around the pipeline periphery for three backfill soils (d50 = 0.15 mm; 0.3 mm; and 0.5 mm) tested. Results indicate that for the range of this experimental study, d50 = 0.15 mm is the best backfill material that provides the most protection for the underneath pipeline. / National Postdoctoral Program for Innovative Talents granted by China Postdoctoral Science Foundation (Grant No. BX20190105) and the Fundamental Research Funds for the Central Universities (Grant No. B200202062).

This study uses dynamic centrifuge tests and three-dimensional (3D), nonlinear finite-difference analyses to: (1) evaluate the effect of soil-cement grid reinforcement on the seismic response of a deep soft soil profile, and (2) to examine the dynamic response of structures supported by shallow foundations on soft clay reinforced by soil-cement grids. The soil profile consisted of a 23-m-thick layer of lightly over-consolidated clay, underlain and overlain by thin layers of dense sand. Centrifuge models had two separate zones for a total of four different configurations: a zone without reinforcement, a zone with a "embedded" soil-cement grid which penetrated the lower dense sand layer and had a unit cell area replacement ratio Ar = 24%, a zone with an embedded grid with Ar = 33%, and a zone with a "floating" grid in the upper half of the clay layer with Ar = 33%. Models were subjected to a series of shaking events with peak base accelerations ranging from 0.005 to 0.54g. The results of centrifuge tests indicated that the soil-cement grid significantly stiffened the site compared to the site with no reinforcement, resulting in stronger accelerations at the ground surface for the input motions used in this study. The response of soil-cement grid reinforced soft soil depends on the area replacement ratio, depth of improvement and ground motion characteristics. The recorded responses of the structures and reinforced soil profiles were used to define the dynamic moment-rotation-settlement responses of the shallow foundations across the range of imposed shaking intensities. The results from centrifuge tests indicated that the soil-cement grids were effective at controlling foundation settlements for most cases; onset of more significant foundation settlements did develop for the weakest soil-cement grid configuration under the stronger shaking intensities which produced a rocking response of the structure and caused extensive crushing of the soil-cement near the edges of the shallow foundations. Results from dynamic centrifuge tests and numerical simulations were used to develop alternative analysis methods for predicting the demands imposed on the soil-cement grids by the inertial loads from the overlying structures and the kinematic loading from the soil profile's dynamic response. / Ph. D.

The effects of basin edge slope on the dynamic response of soil deposits are assessed by using one-dimensional and two-dimensional numerical analyses. 24 basin models having trapezoidal cross section are generated to represent different geometries (basin depth, basin edge slope) and soil type. Harmonic base motions with different predominant periods (Tp) are used in the analyses. The results indicate that, for a constant basin edge slope and a constant ratio of fundamental period of site to the predominant period of base motion (Tn/Tp), the response is almost the same for different soil types, basin depths and base motions. In the sloping edge region, one-dimensional response analysis predictions are found to be conservative compared to two-dimensional analysis predictions / however beyond this region they are unconservatively biased by a factor as high as 1.5. The sloping edge region and the horizontal region of the basin are denoted by normalized distance (ND) values varying from 0 to 1 and 1 to 2 respectively. The critical region where maximum amplification observed falls in the range of ND=1.0 to ND=1.5 for basins having slopes greater than 30o. The lower boundary of the critical region is shifted towards as low as ND=0.2 for basins having slopes less than 30o. For a constant value of Tn/Tp, the increase in the amplification is smooth for basins with gentle slopes as compared to basins with steep slopes for the region where ND~1. For a basin and earthquake couple approaching to resonance state (Tn/Tp=1), the amplification for the region where ND is greater than 1 is found to be as high as 100% of that is found for the region where ND~1.